Plain English Meaning
The air outside the aircraft is warmer or colder than the textbook value used for performance charts and altimeter calculations.
Full Definition
An outside air temperature that differs from the temperature defined by the International Standard Atmosphere (ISA) for a given altitude. The ISA standard is 15 °C (59 °F) at sea level, decreasing by approximately 2 °C per 1,000 feet of altitude gain. Any actual temperature warmer or colder than this standard value at the corresponding altitude is nonstandard.
Context Anchor
Seen in density altitude and aircraft performance discussions, especially when comparing actual weather to standard conditions.